A wide variety of intramedullary and extramedullary bone implants, such as plates, nails or the like are used mainly to treat fractures. A distinction is generally made between implants with a transcutaneous transit and those which are implanted without a transcutaneous transit. The former include, for example, external fixators and the latter nails or plates.
A method is known for determining physical and/or chemical values in connection with an implant. For example, it is desirable to determine the static and dynamic load of implants. A method is known from the “Journal of Biomechanics 34” (2001) pp. 849-857 for arranging a receiver coil for receiving external energy, a measuring circuit, a wire resistance strain gauge, a data converter circuit and a transmitting circuit and coil in a so-called interlocking bone nail. These elements are designed to record the forces acting on the bone. A method is known from “The Journal of Bone and Joint Surgery”, Volume 83-A Supplement 2, Part 1 (2001), pp. 62-65, for installing the wire resistance strain gauge in a knee prosthesis and connecting it by cable to an extracorporeal measuring instrument. A method is known from “Medical Engineering & Physics 22” (2000), pp. 469-479, for installing wire resistance strain gauges, e.g. in the case of a femur strap, and to connect them by cable to an extracorporeal measuring instrument. A method for measuring the force acting on vertebral column implants was published in “SPINE”, Volume 25, no. 23, pp. 2981-2986, and load measurements were also made known in connection with dental prosthesis in “Sensors and Actuators” A 97-98 (2002), pp. 548-556.
In all data recording systems connected to implants the transmission of the measured values externally is required. For this purpose use is made, for example, of wires which connect the measuring unit to a measuring instrument or measured value processing unit. Although such a system is not too restrictive to the wearer of the implant, under certain circumstances, provided that the measuring instrument is fixed comfortably on the outside, the feeding of lines through bones and soft tissues may cause constant irritations and even inflammations. If possible and justifiable in terms of cost, wireless telemetry, i.e. the wireless transmission of measured data extracorporeally, is preferable as the method of choice. Such a wireless transmission is described, for example, in the article in “Medical Engineering & Physics” 22 already referred to.
Inductive or capacitive couplings to suitable magnetic and electrical antennae are used at low frequencies for the transmission path in the near field. If electromagnetic far field antennae are used, this involves high frequencies. The transmission path may either lead from the data recording system in the direction of the implant (“uplink”) or from the implant in the direction of the data recording system (“down-link”). The uplink section is often used to supply energy to the implanted system by inductive coupling.
One disadvantage of telemetry systems of prior art is the considerable attenuation of the electromagnetic waves generated during transit through a metal screen. Additionally the attenuation of electromagnetic waves as they are propagating through free space or tissue or materials of any kind is significant.
A further disadvantage consists in the difficulty in realising the transmitting unit (miniaturisation). The screening has a particularly detrimental effect when high frequencies of over 1 MHz are used for transmitting high data rates. Since implantable telemetry systems are preferably integrated in metal capsules of titanium or implant steel, for reasons of tissue compatibility and economy, telemetry is regarded as problematic.